Glassware that is repetitively washed in automatic dishwashing machines has a tendency to develop a surface cloudiness that is irreversible. The cloudiness often manifests itself as an iridescent film that displays rainbow hues in light reflected from the glass surface. The glass becomes progressively more opaque with repeated washings. This cloudiness is believed to be a type of etching or corrosion of the glass. This same type of corrosion is seen on other articles including china, porcelain, and ceramics.
Corrosion of glass in automatic dishwashers is a well known phenomenon. A paper by D. Joubert and H. Van Daele entitled “Etching of Glassware in Mechanical Dishwashing” in Soap and Chemical Specialties, March, 1971, pp. 62, 64, and 67, discusses the influence of various detergent components, particularly those of an alkaline nature. This subject is also discussed in a paper entitled “The Present Position of Investigations into the Behavior of Glass During Mechanical Dishwashing” presented by Th. Altenschoepfer in April, 1971, at a symposium in Charleroi, Belgium, on “The Effect of Detergents on Glassware in Domestic Dishwashers.” See, also, another paper delivered at the same symposium by P. Mayaux entitled “Mechanism of Glass Attack by Chemical Agents.”
It is believed that the glassware corrosion problem relates to two separate phenomena; the first is corrosion or etching due to the leaching out of minerals from the glass composition itself together with hydrolysis of the silicate network, and the second is deposition and redeposition of silicate material onto the glass. It is a combination of the two that can result in the cloudy appearance of glassware that has been washed repeatedly in automatic dishwashers. This cloudiness often manifests itself in the early stages as an iridescent film that becomes progressively more opaque with repeated washings.
Corrosion inhibitors have been added to automatic dishwashing compositions to reduce the etching or corrosion found on glass. For example, see U.S. Pat. No. 2,447,297 to Wegst et al.; U.S. Pat. No. 2,514,304 to Bacon et al.; U.S. Pat. No. 4,443,270 to Baird et al.; U.S. Pat. No. 4,933,101 to Cilley et al.; U.S. Pat. No. 4,908,148 to Caravajal et al.; U.S. Pat. No. 4,390,441 to Beavan. Zinc has been disclosed for use in preventing glass corrosion. For example, see U.S. Pat. No. 4,917,812 to Cilley; U.S. Pat. No. 3,677,820 to Rutkowski; U.S. Pat. No. 3,255,117 to Knapp; U.S. Pat. No. 3,350,318 to Green; U.S. Pat. No. 2,575,576 to Bacon et al.; U.S. Pat. No. 3,755,180 to Austin; and U.S. Pat. No. 3,966,627 to Gray. Automatic dishwashing detergent compositions incorporating aluminum salts have been disclosed for reducing glass corrosion. See International Publication No. WO 96/36687; U.S. Pat. No. 3,701,736 to Austin et al.; U.S. Pat. No. 5,624,892 to Angevaare et al.; and U.S. Pat. No. 5,624,892 to Angevaare et al.; and U.S. Pat. No. 5,598,506 to Angevaare et al.